1. Field of the Invention
The present invention relates to a coating type optical film, a fabrication method thereof and a liquid crystal display (LCD) using the optical film, and more particularly, to a coating type optical film in which a linear polarizing film is directly coated to form a thin high brightness polarizing plate with an overall thickness of 200 μm. The present invention is also directed to a fabrication method of a coating type optical film and an LCD using the optical film.
2. Description of the Related Art
Today, as we rapidly advance toward an information-oriented society, flat panel displays are required to have excellent characteristics such as slimness, lightness, and low power consumption. In these flat panel displays, since an LCD is excellent in resolution, color display and image quality, it is being actively employed as a monitor of a notebook computer or a desktop computer.
Generally, an LCD is provided with two substrates, each having an electric field generation electrode. The two substrates are arranged such that two surfaces with the electric field generation electrodes face each other. Liquid crystal material is injected into a space between the two substrates. Liquid crystal molecules of the liquid crystal material are aligned by an electric field, which is generated when a voltage is applied to the two electric field generation electrodes, so that light transmittance is changed to display an image.
The general structure of a conventional LCD will be described with reference to the accompanied drawings.
FIG. 1 illustrates structure of a conventional LCD schematically. As shown in FIG. 1, the LCD has a first substrate 110 and a second substrate 120 spaced by a predetermined interval from each other.
The first substrate 110 positioned at the lower side is provided with a thin film transistor T1 including a gate electrode 111, a source electrode 115a and a drain electrode 115b. The thin film transistor T1 further includes an active layer 113 and an ohmic contact layer 114. Here, the gate electrode 111 has an insulating layer 112 thereon.
The thin film transistor T1 is provided with a passivation layer 116 thereon. The passivation layer 116 covers the thin film transistor T1. The passivation layer 116 has a contact hole 116c to expose the drain electrode 115b. Next, the passivation layer 116 has a pixel electrode 117 thereon. The pixel electrode 117 is connected to the drain electrode 115b through the contact hole 116c. 
On the other hand, the second substrate 120 has a black matrix 121 on an inner surface thereof at the position corresponding to the thin film transistor T1. Color filters 122a and 122b of red (R), green (G) and blue (B) are orderly arranged beneath the black matrix 121. A common electrode 123 of transparent conductive material is formed beneath the color filters 122a and 122b. Here, each color of the color filters 122a and 122b corresponds to each pixel electrode 117.
A liquid crystal layer 130 is injected between the pixel electrode 117 and the common electrode 123. The alignment of the liquid crystal molecules of the liquid crystal layer 130 is changed by an electric field generated when a voltage is applied to the pixel electrode 117 and the common electrode 123. Here, although not shown in the drawings, alignment films are formed over the pixel electrode 117 and below the common electrode 123 respectively, to decide the initial alignment of liquid crystal molecules.
On the outer surfaces of the two substrates 110 and 120, i.e., beneath the first substrate 110 and on the second substrate, first and second polarizing plates 118 and 124 are arranged, which transmit only the light that is parallel with the light transmission axis to transform natural light into linearly polarized light. Here, the light transmission axis of the first polarizing plate 118 is perpendicular to that of the second polarizing plate 124.
In FIG. 1, although a structure has been described in which the thin film transistor and the pixel electrode are formed on the lower substrate of the LCD and the color filter and the common electrode are formed on the upper substrate, recently a structure has been suggested in which the color filter and the thin film transistor are formed on the lower substrate. Alternatively, an example in which color filter and common electrode are formed on the lower substrate while thin film transistor and pixel electrode are formed on the upper substrate has also been suggested.
However, since the aforementioned LCDs cannot emit light by themselves, a separate light source is required. Accordingly, a backlight is installed at a backside of the liquid crystal panel or below the first polarizing plate 118 shown in FIG. 1. The light emitted from the backlight is incident to the liquid crystal panel and the amount of the incident light is controlled depending on the alignment of liquid crystal molecules so that image is displayed properly. The LCD used for this purpose is called a transmission type LCD. Here, the pixel electrode 117 and the common electrode 123, which are electric field generation electrodes, are formed of transparent conductive material. The two substrates also have to be of transparent substrates.
The polarizing plate used in the LCD transmits one polarization component of the incident light but absorbs the other component. The absorbed component results in heat loss so that 50% or more brightness loss is caused considering reflection at the surface of the polarizing plate.
Accordingly, a structure has been suggested in which a reflective circular polarizer is employed at the lower portion of the LCD to increase the brightness of the LCD by reducing such a loss.
The reflective circular polarizer transmits one circular polarization component of the incident light and reflects the other component of the incident light. The reflected light of the circular polarization component is again reflected while passing through various optical components placed below the circular polarizer, so that the polarization component is converted to a component that can transmit through the circular polarizer and returns to the original location.
In other words, since the overall amount of the incident light is converted to one polarization component and transmitted theoretically, the loss of light caused by the conventional absorption type linear polarizer can be reduced greatly.
FIG. 2 schematically illustrates the structure of a conventional LCD using an optical film. As shown in FIG. 2, a first polarizing plate 202 that is a linear polarizer is disposed below a liquid crystal panel 201 provided with two substrates each having an electrode on an inner surface thereof, and liquid crystal layer injected between the two substrates. Below the first polarizing plate 202, a phase difference film 203 that converts linear polarized light into circular polarized light and vice versa, and a second polarizing plate 205 that is a circular polarizer are disposed. A compensation film 204 can be further disposed between the phase difference film 203 and the second polarizing plate 205. Below the second polarizing plate 205, a sheet 206 for converging and diffusing light and a backlight 207 are disposed respectively. A third polarizing plate 208 having the light transmission axis perpendicular to that of the first polarizing plate 202 is disposed over the liquid crystal panel 201.
The phase difference film and the compensation film are laminated sequentially by adhesive layers 209a, 209b, 209c, 209d and 210.
The liquid crystal panel 201 of the LCD may be configured to have the same structure as the liquid crystal panel of FIG. 1 or not.
Here, the circular polarizer 205 is made by forming a cholesteric liquid crystal layer 205b on a transparent substrate 205a by using cholesteric liquid crystal.
The cholesteric liquid crystal has a selective reflection characteristic to reflect only light having a particular frequency in the incident light according to the helical pitch. Here, the polarization of the reflected light depends on the rotation direction of the liquid crystal molecules. For example, if liquid crystal molecules have a counterclockwise twisted structure (i.e., left-handed structure), only left-circular polarized light is reflected at the corresponding color.
Then, the pitch size of the cholesteric liquid crystal which light experiences is varied depending on the incident angle of the incident light, so that the wavelength of the reflected light changes. Therefore, since color shift in which the color of the light that is not reflected but transmitted in the incident light changes is caused according to the viewing angle of an observer, a compensation film 204 can be disposed on the circular polarizer 205 so as to compensate for the color shift.
Meanwhile, as illustrated in the drawings, the sheet for converging and diffusing the light from the backlight 207 to the liquid crystal panel 201 can be disposed between the circular polarizer 205 and the backlight 207.
As described above, the optical film gets thicker and has lower brightness due to a circular polarizer, a compensation film, a phase difference film and adhesive layers formed between these films.